Magnesium alloy material and method for producing the same

ABSTRACT

There are provided a magnesium alloy material and a method for producing the magnesium alloy material. In a magnesium (Mg) alloy material (e.g., Mg alloy sheet) having a sheet-shaped portion with a thickness of 1.5 mm or more, when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region, the ratio O F /O C  of the basal plane peak ratio O F  in the surface region to the basal plane peak ratio O C  (degree of orientation of (002) planes) in the internal region satisfies 1.05&lt;O F /O C . A sheet-shaped Mg alloy material is obtained by performing rolling on a twin-roll continuous cast material with multiple passes at a reduction ratio of each pass of 25% or less.

TECHNICAL FIELD

The present invention relates to a magnesium alloy material that is suitable for various members such as parts of transport machines, e.g., automobiles, railway vehicles, and airplanes, parts of bicycles, housings of electric and electronic devices, and other structural members and that is also suitable for constituent materials for the members, and to a method for producing the magnesium alloy material. In particular, the present invention relates to a thick magnesium alloy material having high corrosion resistance and surface roughening resistance.

BACKGROUND ART

Lightweight magnesium alloys having high specific strength and specific rigidity have been investigated as constituent materials for various members such as housings of mobile electric and electronic devices, e.g., cellular phones and laptop computers, parts of automobiles, e.g., wheel covers and paddle shifts, parts of railway vehicles, and parts of bicycles, e.g., frames. Members composed of a magnesium alloy are mainly formed of cast materials (AZ91 alloy of the American Society for Testing and Materials (ASTM) standard) by a die casting process or a thixomolding process. In recent years, press-formed materials obtained by performing press forming on a sheet composed of a wrought magnesium alloy such as AZ31 alloy of the ASTM standard have been used. Patent Literature 1 discloses that a continuous cast material composed of a magnesium alloy such as AZ91 alloy is produced by a twin-roll casting process, and a rolled sheet obtained by rolling the continuous cast material is subjected to press forming.

CITATION LIST Patent Literature

Patent Literature 1: International Publication No. 2006/003899

SUMMARY OF INVENTION Technical Problem

Focusing on the lightweight of magnesium alloys, a relatively thin sheet having a thickness of 1 mm or less has been investigated as a raw material for plastic formed materials such as press formed materials. However, with the expansion of usage of magnesium alloys, the development of not only the above-described thin sheet but also a thick sheet, specifically, a thick sheet having a thickness of 1.5 mm or more has been demanded focusing on the specific strength and specific rigidity. Such a thick magnesium alloy sheet, a method for producing the thick magnesium alloy sheet, and a plastic formed material such as a press formed material produced using the thick magnesium alloy sheet have not been sufficiently studied.

A thick magnesium alloy sheet is obtained by employing a die casting process or a thixomolding process. However, in cast materials such as die cast materials, internal defects such as cavities are easily formed, and furthermore the composition and structure tend to become uneven. For example, additive element components are locally highly concentrated and crystal grains are randomly oriented. Therefore, cast materials such as die cast materials have low corrosion resistance compared with plastic formed materials such as rolled materials. In addition, cast materials such as die cast materials have poor plastic formability due to their internal defects and thus are not suitable as raw materials for plastic forming.

Accordingly, it is an object of the present invention to provide a thick magnesium alloy material having high corrosion resistance and surface roughening resistance and a thick magnesium alloy material subjected to plastic forming. It is another object of the present invention to provide a method for producing a thick magnesium alloy material having high corrosion resistance and surface roughening resistance.

Solution to Problem

A magnesium alloy material subjected to plastic forming (primary forming) such as rolling is excellent in terms of mechanical properties such as strength, hardness, and toughness, corrosion resistance, and plastic formability compared with die cast materials and thixomolded materials even if they have the same composition. This is because, in such a magnesium alloy material, formation of defects during casting is reduced and the crystal is made finer. A magnesium alloy material obtained by performing plastic forming (secondary forming) such as press forming on the magnesium alloy material subjected to the primary forming is also excellent in terms of mechanical properties and corrosion resistance. In particular, when a continuous cast material produced by a continuous casting process such as a twin-roll casting process is used as a raw material for primary formed materials, the continuous cast material is excellent in terms of plastic formability because segregation and generation of coarse impurities in crystal and precipitated impurities are reduced compared with die cast materials. Therefore, the inventors of the present invention have produced a thick magnesium alloy sheet having a thickness of 1.5 mm or more by rolling a continuous cast material under various conditions. As a result, the inventors have found the following. A magnesium alloy sheet produced under certain conditions is thick and has high corrosion resistance. Furthermore, when plastic forming such as press forming or bending is performed, the resultant plastic formed material has a small number of small projections and depressions on its surface, that is, has a smooth surface (e.g., a beautiful surface with gloss), which means that such a plastic formed material has high surface roughening resistance. The present invention is based on the above findings.

A magnesium alloy material of the present invention is composed of a magnesium alloy and includes a sheet-shaped portion having a thickness of 1.5 mm or more. The sheet-shaped portion satisfies the following orientation,

[Orientation]

when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region,

X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the surface region are respectively defined as I_(F)(002), I_(F)(100), I_(F)(101), I_(F)(102), I_(F)(110), and I_(F)(103),

X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the internal region are respectively defined as I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103),

a degree of orientation of the (002) plane in the surface region, which is expressed as I_(F)(002)/{I_(zf)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}, is defined as a basal plane peak ratio O_(F), and

a degree of orientation of the (002) plane in the internal region, which is expressed as I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}, is defined as a basal plane peak ratio O_(C),

a ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region satisfies 1.05<O_(F)/O_(C).

The magnesium alloy material of the present invention can be produced, for example, by the following production method of the present invention. A method for producing a magnesium alloy material according to the present invention is a method for producing a magnesium alloy material by rolling a raw material composed of a magnesium alloy, the method including a preparation step and a rolling step below.

Preparation step: a step of preparing a sheet-shaped raw material obtained by subjecting a molten magnesium alloy to continuous casting by a twin-roll casting process

Rolling step: a step of rolling the raw material with multiple passes to produce a sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more

In the rolling step, the reduction ratio of each of the passes is 25% or less.

The reduction ratio (%) herein is defined as {(thickness t_(b) of raw material before Rolling−thickness t_(a) of raw material after rolling)/thickness t_(b) of raw material before rolling}×100.

According to the production method of the present invention, rolling with multiple passes can be favorably performed by employing, as a raw material, a continuous cast material in which defects and impurities in crystal and precipitated impurities, from which cracking or the like is caused, and segregation are present in a small amount or are substantially not present. Furthermore, by performing rolling with multiple passes while relatively decreasing the reduction ratio of each pass, plastic forming by rolling is sufficiently imparted to a surface portion of a rolled material compared with an internal portion of the rolled material. In other words, the surface structure and internal structure of the rolled material can be differentiated from each other by repeatedly performing rolling at a relatively low reduction ratio. Therefore, the production method of the present invention provides a magnesium alloy material (typically a rolled sheet (one form of the magnesium alloy material of the present invention)) constituted by a structure in the surface region and a structure in the internal region which are different from each other. More specifically, the structure in the surface region is a texture in which basal planes of magnesium alloy crystals are mainly oriented so as to be parallel to the rolling direction (a direction in which a raw material to be rolled travels) by sufficiently providing plastic forming by rolling (a texture in which c axes of the magnesium alloy crystals are oriented so as to be orthogonal to the rolling direction). The structure in the internal region is a structure in which the basal planes of the magnesium alloy crystals are oriented more randomly than those of the surface region.

When the magnesium alloy material of the present invention is the above-described particular rolled sheet (i.e., when the entire magnesium alloy material of the present invention is constituted by the sheet-shaped portion), the structure in the surface region is a texture having a particular orientation, more specifically, a texture in which (002) planes, which are basal planes of magnesium alloy crystals, are strongly oriented and the structure in the internal region is a structure in which (002) planes are oriented more weakly than those in the surface region. The texture in which (002) planes are strongly oriented is one of indices indicating that deformation due to plastic forming is sufficiently applied during plastic forming such as rolling. As the plastic forming such as rolling is sufficiently performed, the crystal grain size of a magnesium alloy tends to decrease, which increases the total area of crystal grain boundaries. As a result, the ratio of impurity elements present relative to the crystal grain boundaries is relatively decreased and thus the magnesium alloy material of the present invention having the above-described particular structure has high corrosion resistance. In particular, when the surface region exposed to an outside atmosphere has a structure finer than that of the internal region, higher corrosion resistance is achieved. Therefore, the magnesium alloy material of the present invention is thick and has high corrosion resistance. Furthermore, the magnesium alloy material is constituted by different structures between the surface region and the internal region as described above, whereby the magnesium alloy material has different properties (e.g., mechanical properties such as hardness, strength, impact resistance, and toughness, corrosion resistance, and vibration resistance) between the surface region and the internal region. By using such a difference in properties, the magnesium alloy material of the present invention can be expected to be used for various members and as a raw material for the various members. In addition, the magnesium alloy material of the present invention has good plastic formability such as press formability or bendability because the degree of orientation of the basal planes ((002) planes) in the internal region is small (the degree of orientation density in a texture is small). Thus, the magnesium alloy material can be suitably used as a raw material to be subjected to plastic forming such as press forming or bending. The surface region is constituted by a fine crystalline structure. Therefore, even if plastic forming such as press forming is performed, large projections and depressions are not easily formed on the surface of a raw material and a plastic formed material (one form of the magnesium alloy material of the present invention) having a smooth surface is provided. Accordingly, the magnesium alloy material of the present invention has high surface roughening resistance. The resultant plastic formed material also has good surface texture.

According to an embodiment of the magnesium alloy material of the present invention, when an average crystal grain size in the surface region is defined as D_(F) and an average crystal grain size in the internal region is defined as D_(C), a ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region satisfies 1.5<D_(C)/D_(F).

According to the above embodiment, the crystal grain size in the internal region is larger than that in the surface region. In other words, the crystal grain size in the surface region is sufficiently smaller than that in the internal region, which increases the length of the crystal grain boundaries as described above and thus high corrosion resistance is achieved. According to the above embodiment, since the surface region is constituted by a fine crystalline structure, good plastic formability and high surface roughening resistance are achieved. Furthermore, since the crystal grain size in the internal region is larger than that in the surface region, and thus high heat resistance is achieved.

According to an embodiment of the magnesium alloy material of the present invention, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies H_(C)/H_(F)<0.85.

According to the above embodiment, the Vickers hardness in the internal region is lower than that in the surface region. In other words, the Vickers hardness in the surface region is sufficiently higher than that in the internal region and thus high wear resistance is achieved.

The magnesium alloy material of the present invention can be composed of a magnesium alloy containing various additive elements (balance: Mg and impurities). In particular, an alloy containing additive elements in a high concentration, specifically, a magnesium alloy containing additive elements in a total content of 5.0 mass % or more has good mechanical properties such as strength and hardness, high corrosion resistance, flame resistance, heat resistance, and the like, though depending on the types of additive elements.

Specific examples of the additive elements include at least one element selected from Al, Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Li, Zr, Ce, Ni, Au, and rare-earth elements (except for Y and Ce). An example of the impurities is Fe.

A Mg—Al series alloy containing Al has high corrosion resistance and also has good mechanical properties such as strength and hardness. Therefore, according to an embodiment of the magnesium alloy material of the present invention, the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less. As the Al content increases, the above effects tend to increase. The Al content is preferably 7 mass % or more and more preferably 7.3 mass % or more. The upper limit of the Al content is 12 mass % and preferably 11 mass % because the plastic formability degrades when the Al content exceeds 12 mass %. In particular, a magnesium alloy containing 8.3 mass % to 9.5 mass % of Al provides high strength and corrosion resistance. The total content of elements other than Al is 0.01 mass % or more and 10 mass % or less and preferably 0.1 mass % or more and 5 mass % or less.

Specific examples of the Mg—Al series alloy include AZ series alloys (Mg—Al—Zn series alloys, Zn: 0.2 mass % to 1.5 mass %, such as AZ 31 alloy, AZ61 alloy, and AZ91 alloy) in the ASTM standard, AM series alloys (Mg—Al—Mn series alloys, Mn: 0.15 mass % to 0.5 mass %), AS series alloys (Mg—Al—Si series alloys, Si: 0.01 mass % to 20 mass %), Mg—Al—RE (rare-earth element) series alloys, AX series alloys (Mg—Al—Ca series alloys, Ca: 0.2 mass % to 6.0 mass %), and AJ series alloys (Mg—Al—Sr series alloys, Sr: 0.2 mass % to 7.0 mass %). Examples of the alloy containing 8.3 mass % to 9.5 mass % of Al include Mg—Al—Zn series alloys further containing 0.5 mass % to 1.5 mass % of Zn, e.g., AZ91 alloy.

A magnesium alloy containing at least one element selected from Y, Ce, Ca, Si, Sn, and rare-earth elements (except for Y and Cc) in a total content of 0.001 mass % or more and preferably 0.1 mass % or more and 5 mass % or less, the balance being Mg and impurities, provides high heat resistance and flame resistance. When rare-earth elements are contained, the total content is preferably 0.1 mass % or more. In particular, when Y is contained, the content is preferably 0.5 mass % or more.

Advantageous Effects of Invention

A magnesium alloy material according to the present invention is a thick magnesium alloy material having high corrosion resistance and surface roughening resistance. In a method for producing a magnesium alloy material according to the present invention, a thick magnesium alloy material having high corrosion resistance and surface roughening resistance can be produced.

DESCRIPTION OF EMBODIMENTS

The present invention will now be further described in detail.

[Magnesium Alloy Material] (Composition)

The magnesium alloy material of the present invention is composed of a magnesium alloy containing 50 mass % or more of Mg and, typically, the additive elements described above.

(Form)

A sheet-shaped portion in the magnesium alloy material of the present invention means a portion having a pair of surfaces parallel to each other with a substantially uniform interval (distance) between the surfaces, that is, a portion having a uniform thickness. The magnesium alloy material of the present invention has a sheet-shaped portion in at least part thereof. When this is satisfied, the magnesium alloy material may have a form having a portion with a thickness locally different from that of the other portions through a process such as a cutting process, e.g., a form including, for example, a boss joined thereto, a form having a groove, and a form having a through-hole that connects the top and the bottom.

A typical form of the magnesium alloy material of the present invention having the sheet-shaped portion is a form (magnesium alloy sheet) in which the entire magnesium alloy material has a sheet-like shape. The shape (planar shape) of the magnesium alloy sheet may be a rectangular shape, a circular shape, or the like. The magnesium alloy sheet may be in the form of a coil stock obtained by coiling a continuous long sheet or in the form of a short sheet having a predetermined length and shape. The magnesium alloy sheet may have various forms in accordance with the production process. Examples of the forms include a rolled sheet, a heat treated sheet and a leveled sheet respectively obtained by performing a heat treatment and leveling described below on the rolled sheet, and a polished sheet and a coated sheet respectively obtained by performing polishing and coating on the rolled sheet, heat-treated sheet, or leveled sheet.

The magnesium alloy material of the present invention may also be a formed product obtained by performing plastic forming (secondary forming) such as press forming, e.g., bending and drawing on the magnesium alloy sheet or a partly formed material having a plastic formed portion, which is obtained by partly performing plastic forming on the magnesium alloy sheet (herein, at least part of the formed product or partly formed material is the sheet-shaped portion). The formed product may be a box or frame having a U-shaped cross section that includes a top (bottom) and a sidewall extending perpendicularly from the periphery of the top (bottom) or a covered tube that includes a discoidal top and a cylindrical sidewall. At least the top corresponds to the sheet-shaped portion. The form of the magnesium alloy material can be selected in accordance with desired applications.

(Thickness)

In the magnesium alloy material of the present invention, the sheet-shaped portion has a thickness of 1.5 mm or more. Any thickness can be selected from a thickness of 1.5 mm or more in accordance with desired applications. Herein, to increase the thickness of the sheet-shaped portion, the thickness of a cast material serving as a raw material needs to be also increased. If the thickness of the cast material is increased, the rollability is degraded due to the above-described defects or the like. Therefore, the thickness of the sheet-shaped portion is preferably 10 mm or less and particularly preferably 5 mm or less for the purpose of producing a thick rolled sheet (one form of the magnesium alloy material of the present invention) with high productivity.

When the magnesium alloy material of the present invention is the formed product or the partly formed material, the structure and mechanical properties of the magnesium alloy sheet serving as a raw material for plastic forming are substantially maintained in a portion (e.g., the sheet-shaped portion) that undergoes less deformation during the plastic forming

(Structure) <Orientation>

In the magnesium alloy material of the present invention, at least the surface region in the above sheet-shaped portion is constituted by a structure having a texture of basal planes and the internal region is constituted by a structure having a small degree of orientation of basal planes. When the surface region exposed to an outside atmosphere is constituted by a structure in which (002) planes are strongly oriented, higher corrosion resistance is achieved as described above. As the difference in the degree of orientation between the surface region and the internal region increases, the corrosion resistance, surface hardness, and surface roughening resistance are expected to be improved. However, if the difference in the degree of orientation is excessively increased, it becomes difficult to uniformly perform plastic forming such as press forming. Therefore, the above basal plane peak ratio O_(F)/O_(C) preferably satisfies O_(F)/O_(C)≦1.2.

<Average Crystal Grain Size>

In a typical form of the magnesium alloy material of the present invention, the crystal grain size in the internal region is larger than that in the surface region. In this form, the internal region has high heat resistance and the surface region having a relatively small crystal grain size has high corrosion resistance and hardness as described above. In particular, when the surface region is constituted by a relatively fine structure, high hardness is achieved and thus high wear resistance is achieved. This makes it difficult to form scratches and provides good surface texture. Therefore, the magnesium alloy material of the present invention is expected to be suitably used for structural materials and the like that require durability. As the difference in the average crystal grain size between the surface region and the internal region increases, the corrosion resistance, surface roughening resistance, and surface hardness are expected to be improved. However, if the difference in the average crystal grain size is excessively increased, it becomes difficult to uniformly perform plastic forming such as press forming. Therefore, the above ratio D_(C)/D_(F) of the average crystal grain sizes preferably satisfies D_(C)/D_(F)≦2.0.

In the case where a thick sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more is produced by performing rolling as described above, there are limitations to achievement of a uniform and fine grain size across the entire region in the thickness direction. In the magnesium alloy material of the present invention, the average crystal grain sizes in the surface region and internal region are 3.5 μm or more. However, since the plastic formability tends to improve as the crystal grain size decreases, both the average crystal grain sizes in the surface region and internal region of the sheet-shaped portion are preferably 20 μm or less and particularly preferably 10 μm or less. The average crystal grain size varies depending on the reduction ratio and the heating temperature of a raw material in the rolling step, and tends to decrease as the reduction ratio increases and the heating temperature decreases.

(Mechanical Properties)

The magnesium alloy material of the present invention has better mechanical properties such as strength, hardness, and toughness than cast materials such as die cast materials because rolling is performed. For example, the Vickers hardness in the surface region is higher than that in the internal region as described above. As the difference in the Vickers hardness between the surface region and the internal region increases, the surface hardness relatively increases. However, if the difference in the Vickers hardness is excessively increased (the surface hardness is excessively increased), press formability is degraded. Therefore, the ratio H_(C)/H_(F) of the Vickers hardnesses (Hv) preferably satisfies 0.7≦H_(C)/H_(F). The absolute value of the Vickers hardness tends to increase as the content of additive elements increases, though depending on the rolling conditions such as the reduction ratio and the heating temperature of a raw material. When the magnesium alloy material of the present invention is a plastic formed material (formed product) or a partly formed material, the hardness tends to be further increased by work hardening.

(Other Structures)

By subjecting at least part of the surface of the magnesium alloy material of the present invention to an anti-corrosion treatment such as a chemical conversion treatment or an anodic oxidation treatment to form an anti-corrosion layer, higher corrosion resistance is achieved. Furthermore, by coating at least part of the surface of the magnesium alloy material of the present invention to form a coating layer, the design and commercial value are improved.

[Production Method]

Each step of the above-described production method of the present invention will now be further described in detail.

(Preparation Step) <Casting>

In the production method of the present invention, a continuous cast material is used as a starting material. In a continuous casting process, rapid solidification can be performed. Therefore, segregation and formation of oxides can be reduced even when additive elements are contained in a large amount, which can suppress the generation of coarse impurities in crystal and precipitated impurities having a size of more than 10 μm from which cracking may be caused. Thus, a cast material having good plastic formability such as rollability can be produced. In a continuous casting process, a long cast material can also be produced in a continuous manner. A long material produced by the continuous casting process can be used as a raw material for rolling. When a long raw material is used, a long rolled material can be produced. Examples of the continuous casting process include a twin-roll process, a twin-belt process, and a belt-and-wheel process. A twin-roll process or a twin-belt process is suitable for the production of sheet-shaped cast materials, and a twin-roll process is particularly suitable. A continuous cast material produced by the casting process described in Patent Literature 1 is preferably used. The thickness, width, and length of a cast material can be suitably selected so that a desired rolled material (rolled sheet) is obtained. The thickness of the cast material is preferably 10 mm or less and particularly preferably 5 mm or less because segregation is easily caused in an excessively thick cast material. When the obtained continuous cast material is used as a long material, such a continuous cast material is coiled in a cylindrical shape because the continuous cast material is easily transferred to the next step. When the cast material is coiled while a start-of-coiling portion of the cast material is heated to about 100° C. to 200° C., even alloys such as AZ91 alloy which contain additive elements in a large amount and easily cause cracking are easily bent. Even in the case of a small coiling diameter, the cast material can be coiled without being cracked. The obtained continuous cast material can be cut into sheet materials having a desired length to obtain a raw material for rolling. In this case, a rolled material (rolled sheet) having a desired length is obtained.

<Solution Treatment>

By performing a solution treatment before the cast material is rolled, the composition of the cast material can be homogenized and elements such as Al can be sufficiently dissolved to improve the toughness. The solution treatment is performed at a heating temperature of 350° C. or more and particularly 380° C. or more and 420° C. or less for a holding time of 1 hour or more and 40 hours or less. In the case of Mg—Al series alloys, the holding time is preferably increased as the content of Al increases. In a cooling step from the heating temperature after the holding time, the precipitation of coarse precipitates can be suppressed by increasing the cooling rate (preferably 50° C./min or more) using accelerated cooling such as water cooling or air blast cooling.

<Rolling>

The cast material or the material subjected to a solution treatment is rolled with multiple passes. At least one pass is preferably performed by warm rolling which is performed while a raw material (a cast material, a material subjected to a solution treatment, or a worked material being subjected to rolling) is heated to 150° C. or more and 400° C. or less or by hot rolling. By heating the raw material to the above temperature, cracking or the like is not easily caused during the rolling even if the reduction ratio per pass is increased. As the temperature is increased, the formation of cracks is reduced. By setting the temperature to be 400° C. or less, the degradation caused by seizing of a surface of the raw material and the thermal degradation of a reduction roll can be suppressed. Therefore, the heating temperature is preferably 350° C. or less, more preferably 300° C. or less, and particularly preferably 150° C. or more and 280° C. or less. In addition to the raw material, the reduction roll may also be heated. The heating temperature of the reduction roll is, for example, 100° C. to 250° C.

In the production method of the present invention, the reduction ratio of each pass is set to be 25% or less. By performing rolling with multiple passes at a relatively low reduction ratio, plastic forming can be imparted particularly to the surface of the raw material in a concentrated manner. The reduction ratio of each pass can be suitably selected in the range of 25% or less. However, if the reduction ratio is excessively low, the number of passes to achieve a desired thickness is increased, which decreases the productivity. Thus, the reduction ratio of each pass is preferably 10% or more.

The conditions such as the heating temperature of a raw material, the temperature of a reduction roll, and the reduction ratio may be changed for each pass. Therefore, the reduction ratio of each pass may be the same or different. An intermediate heat treatment may be performed between the passes. By performing the intermediate heat treatment, the strain and residual stress introduced into the raw material before the intermediate heat treatment can be removed or reduced to allow ease of rolling after the intermediate heat treatment. The intermediate heat treatment can be performed at a heating temperature of 150° C. to 350° C. (preferably 300° C. or less, more preferably 250° C. to 280° C.) for a holding time of 0.5 to 3 hours. After the rolling, a final heat treatment may be performed under the above conditions. Furthermore, the rolling is easily performed by suitably using a lubricant because the frictional resistance during rolling can be reduced and the seizing of the raw material can be prevented.

In addition, the edge of the cast material before rolling may be trimmed to prevent the extension of cracks, which may present at the edge, during rolling. Such trimming may be performed to appropriately adjust the width during rolling or after rolling.

<Other Processes> <<Polishing>>

After the rolling, polishing may be performed. The polishing is performed in order to remove or reduce the lubricant used during the rolling and the scratches and oxide films present on the surface of the rolled material. The polishing is preferably performed with a grinding belt because even a long material can be easily polished in a continuous manner. The polishing is preferably performed by a wet process to prevent scattering of powder.

<<Leveling>>

After the rolling or after the polishing, leveling may be performed. The leveling is performed in order to improve the flatness and precisely perform plastic forming such as press forming. In the leveling, a roll leveler including a plurality of rollers disposed in a staggered manner can be suitably used. The leveling may be performed while the raw material is heated to, for example, 100° C. to 300° C. and particularly 150° C. to 280° C. (warm leveling).

<<Plastic Forming>>

When the magnesium alloy material of the present invention is processed into a formed product or a partly formed material having a plastic formed portion, such a formed product or a partly formed material can be produced by a production method that includes a plastic forming step of performing plastic forming such as press forming on at least part of the raw material (the above-described rolled material, polished material, or leveled material) subjected to the above rolling step. The plastic forming is preferably performed in the temperature range of 200° C. to 300° C. to improve the plastic formability of the raw material. A heat treatment may be performed after the plastic forming in order to remove the strain and residual stress introduced during the plastic forming and improve the mechanical properties. The heat treatment is performed at a heating temperature of 100° C. to 300° C. for a heating time of about 5 to 60 minutes.

<<Surface Treatment>>

When the magnesium alloy material of the present invention includes the above-described anti-corrosion layer or coating layer, the production can be performed by a production method that includes a surface treating step of performing an anti-corrosion treatment or coating on at least part of the raw material subjected to the rolling step or at least part of the raw material subjected to the plastic forming step. Furthermore, at least one selected from hairline finish, diamond cutting, shot blasting, etching, and spin cutting may be performed on at least part of the raw material. Such a surface treatment is performed in order to improve the corrosion resistance and a mechanical protective function and improve the design, metal texture, and commercial value.

A specific embodiment of the present invention will now be further described based on Test Examples.

[Test Example]

Raw materials composed of magnesium alloys having the following compositions were subjected to rolling under various conditions to produce magnesium alloy sheets having a thickness of 1.5 mm or more. The orientation, crystal grain size, and Vickers hardness of the magnesium alloy sheets were measured.

In this test, a magnesium alloy sheet composed of a magnesium alloy (Mg—9.0 mass % Al—0.6 mass % Zn) having a composition equivalent to that of AZ91 alloy and a magnesium alloy sheet composed of a magnesium alloy (Mg—3.1 mass % Al—0.7 mass % Zn) having a composition equivalent to that of AZ31 alloy were produced.

Long cast sheets (thickness 4.5 mm (4.50 to 4.51 mm)×width 320 mm) were produced from the above magnesium alloys having the above compositions by a twin-roll continuous casting process. The long cast sheets were temporarily coiled to produce cast coil stocks. Each of the cast coil stocks was subjected to a solution treatment at 400° C. for 24 hours. A raw material obtained by uncoiling the solid solution coil stock subjected to the solution treatment was rolled with multiple passes under the rolling conditions shown in Table I to produce a rolled material (magnesium alloy sheet) having a thickness of 2.0 mm (2.00 mm to 2.01 mm) or 1.5 mm. Each pass was performed by warm rolling (the heating temperature of a raw material: 250° C. to 280° C., the temperature of a reduction roll: 100° C. to 250° C.). The thickness of the cast material, the thickness of a worked material being subjected to rolling, and the thickness of the obtained magnesium alloy sheet were determined to be the average of three thicknesses in total at the central part in the width direction of a sheet to be measured and two points located 50 mm from the edges in the width direction.

TABLE I Thickness of each pass (mm) Sample Before Reduction ratio of each pass (%) Composition No. rolling 1 2 3 4 5 6 1 2 3 4 5 6 AZ91 A 4.50 3.25 2.50 2.00 — — — 27.8 23.1 20.0 — — — B 4.51 3.61 2.88 2.40 2.00 — — 20.0 20.2 16.7 16.7 — — C 4.51 3.86 3.26 2.78 2.35 2.00 — 14.4 15.5 14.7 15.5 14.9 — D 4.50 3.20 2.32 1.85 1.50 — — 28.9 27.5 20.3 18.9 — — E 4.51 3.45 2.69 2.12 1.70 1.50 — 23.5 22.0 21.2 19.8 11.8 — F 4.51 3.75 3.11 2.55 2.08 1.75 1.50 16.9 17.1 18.0 18.4 15.9 14.3 AZ31 G 4.51 3.25 2.51 2.01 — — — 27.9 22.8 19.9 — — — H 4.50 3.60 2.88 2.41 2.00 — — 20.0 20.0 16.3 17.0 — — I 4.51 3.85 3.26 2.77 2.35 2.01 — 14.6 15.3 15.0 15.2 14.5 — J 4.50 3.21 2.32 1.86 1.50 — — 28.7 27.7 19.8 19.4 — — K 4.50 3.46 2.70 2.11 1.71 1.50 — 23.1 22.0 21.9 19.0 12.3 — L 4.51 3.76 3.10 2.55 2.09 1.75 1.50 16.6 17.6 17.7 18.0 16.3 14.3

[Orientation]

Each of the obtained magnesium alloy sheets was analyzed by X-ray diffraction to determine the ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region. Table II shows the results. The basal plane peak ratio O_(F) in the surface region was measured by performing X-ray diffraction on the surface of the magnesium alloy sheet. The basal plane peak ratio O_(C) in the internal region was measured by chemically removing a region (surface region) having ¼ the thickness of the magnesium alloy sheet in a thickness direction from a surface of the magnesium alloy sheet to expose the inside portion and then performing X-ray diffraction on the exposed surface. The peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in each of the regions were measured to determine the ratio O_(F)/O_(C).

Basal plane peak ratio O_(F): I_(F)(002)/{I_(F)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}

Basal plane peak ratio O_(C): I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110) +I_(C)(103)}

[Average Crystal Grain Size]

The average crystal grain sizes (μm) in the internal region and surface region of each of the obtained magnesium alloy sheets were measured in conformity with “Steels—Micrographic determination of the apparent grain size JIS G 0551 (2005)”. Sections (cross section and longitudinal section) of the magnesium alloy sheet in the thickness direction were taken and observed with an optical microscope (400 times) to determine the average crystal grain size in each of three fields of view (the total number of fields of view in each region: 6) in the sections of the surface region (a region having ¼ the thickness from a surface in the thickness direction) and internal region (a remaining region obtained by removing the surface region). Table II shows the average (D_(F)) of the average crystal grain sizes in the six fields of view in total in the surface region and the average (D_(C)) of the average crystal grain sizes in the six fields of view in total in the internal region. The ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region was also determined. Table II shows the results.

[Vickers Hardness]

The Vickers hardnesses (Hv) in the internal region and surface region of each of the obtained magnesium alloy sheets were measured. As in the case of the measurement of the average crystal grain size, sections (cross section and longitudinal section) of the magnesium alloy sheet in the thickness direction were taken. The Vickers hardness H_(F) in the surface region was measured by pressing an indenter against each of the sections of the surface region. The Vickers hardness H_(C) in the internal region was measured by pressing an indenter against each of the sections of the internal region. Table II shows the average (H_(F)) of the Vickers hardnesses in the sections of the surface region and the average (H_(C)) of the Vickers hardnesses in the sections of the internal region. The ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region was also determined. Table II shows the results.

[Corrosion Test]

The corrosion resistance of each of the obtained magnesium alloy sheets was measured. A test piece (the thickness was the same as that of the obtained magnesium alloy sheet) was prepared in conformity with JIS Z 2371 (2000). A salt-spray test was performed for 96 hours to measure the corrosion weight loss (mg/cm²) after the test. Table II shows the results.

TABLE II Surface roughness Corrosion Crystal Vickers Ra in bent weight Basal plane grain size hardness portion loss in peak ratio Surface/ (μm) Internal/ (Hv) Internal/ after press salt-spray Sample Surface Internal Internal Surface Internal Surface Surface Internal Surface forming test Composition No. O_(F) O_(C) O_(F)/O_(C) D_(F) D_(C) D_(C)/D_(F) H_(F) H_(C) H_(C)/H_(F) (μm) (mg/cm²) AZ91 A 0.861 0.841 1.02 5.2 5.1 1.02 88 89 1.01 0.65 0.31 B 0.870 0.814 1.07 4.9 7.5 1.53 90 75 0.83 0.49 0.13 C 0.883 0.805 1.10 4.8 8.8 1.83 89 73 0.82 0.49 0.12 D 0.878 0.860 1.02 5.2 5.1 1.04 90 89 0.99 0.56 0.33 E 0.880 0.836 1.052 5.0 7.6 1.52 89 75 0.84 0.52 0.12 F 0.882 0.820 1.08 4.8 8.5 1.77 91 75 0.82 0.47 0.11 AZ31 G 0.711 0.691 1.03 6.3 6.4 1.07 68 66 0.97 1.09 0.62 H 0.720 0.682 1.06 5.3 8.1 1.53 70 59 0.84 0.47 0.40 I 0.731 0.668 1.09 4.9 8.9 1.82 69 57 0.83 0.44 0.41 J 0.721 0.718 1.00 6.4 6.5 1.05 67 66 0.99 1.02 0.65 K 0.729 0.685 1.06 5.2 7.9 1.52 70 58 0.83 0.46 0.42 L 0.734 0.671 1.09 4.8 9.0 1.88 71 57 0.80 0.44 0.40

As is clear from Tables I and II, by rolling a continuous cast material with multiple passes at a reduction ratio of 25% or less per pass, there is provided a thick magnesium alloy sheet (magnesium alloy material) which has a thickness of 1.5 mm or more and in which the structure (basal plane peak ratio) in the internal region in the thickness direction and the structure (basal plane peak ratio) in the surface region are different from each other. In this magnesium alloy sheet, the mechanical properties in the internal region are different from the mechanical properties in the surface region.

When each of the obtained magnesium alloy sheets was subjected to press forming (the heating temperature of the magnesium alloy sheet: 250° C. to 270° C.), all the samples were successfully press formed. In the measurement of the structure of a flat portion in each of the press formed materials, the structure of a flat portion was substantially the same as that of the magnesium alloy sheet before press forming, and the basal plane peak ratio and average crystal grain size were substantially equivalent to those of the magnesium alloy sheet before press forming. Furthermore, the surface roughness Ra in a bent portion of the press formed material was measured. Table II shows the results, which make it clear that samples having different structures between the surface region and the internal region, specifically, sample Nos. B, C, E, F, H, I, K, and L constituted by a structure in which the grain size in the surface region is small have a small surface roughness Ra of about 0.5 μm or less and thus have a smooth surface.

It was confirmed from the above test results that a magnesium alloy material which has a thick sheet-shaped portion with a thickness of 1.5 mm or more and in which the structure of the sheet-shaped portion varies in the thickness direction and has a particular orientation had high surface roughening resistance. It was also confirmed that, when the magnesium alloy material had a surface region constituted by a relatively fine structure, the magnesium alloy material had high corrosion resistance.

The above-described embodiments can be suitably modified without departing from the scope of the present invention and are not limited to the above configurations. For example, the composition of the magnesium alloy, the thickness and shape of the magnesium alloy material, and the reduction ratio of each pass and the number of passes in the rolling step can be suitably changed.

INDUSTRIAL APPLICABILITY

The magnesium alloy material of the present invention can be suitably used for members in various fields that require corrosion resistance and wear resistance, such as parts of automobiles, parts of railway vehicles, parts of airplanes, parts of bicycles, and parts of electric and electronic devices; constituent materials for the members; and bags and raw materials thereof The method for producing a magnesium alloy material according to the present invention can be suitably used for the production of the above magnesium alloy material of the present invention. 

1. A magnesium alloy material composed of a magnesium alloy, comprising a sheet-shaped portion, wherein the sheet-shaped portion has a thickness of 1.5 mm or more, and the sheet-shaped portion satisfies the following orientation, [Orientation] when a region having ¼ the thickness of the sheet-shaped portion in a thickness direction from a surface of the sheet-shaped portion is defined as a surface region and a remaining region is defined as an internal region, X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the surface region are respectively defined as I_(F)(002), I_(F)(100), I_(F)(101), I_(F)(102), I_(F)(110), and I_(F)(103), X-ray diffraction peak intensities of a (002) plane, a (100) plane, a (101) plane, a (102) plane, a (110) plane, and a (103) plane in the internal region are respectively defined as I_(C)(002), I_(C)(100), I_(C)(101), I_(C)(102), I_(C)(110), and I_(C)(103), a degree of orientation of the (002) plane in the surface region, which is expressed as I_(F)(002)/{I_(F)(100)+I_(F)(002)+I_(F)(101)+I_(F)(102)+I_(F)(110)+I_(F)(103)}, is defined as a basal plane peak ratio O_(F), and a degree of orientation of the (002) plane in the internal region, which is expressed as I_(C)(002)/{I_(C)(100)+I_(C)(002)+I_(C)(101)+I_(C)(102)+I_(C)(110)+I_(C)(103)}, is defined as a basal plane peak ratio O_(C), a ratio O_(F)/O_(C) of the basal plane peak ratio O_(F) in the surface region to the basal plane peak ratio O_(C) in the internal region satisfies 1.05<O_(F)/O_(C).
 2. The magnesium alloy material according to claim 1, wherein, when an average crystal grain size in the surface region is defined as D_(F) and an average crystal grain size in the internal region is defined as D_(C), a ratio D_(C)/D_(F) of the average crystal grain size D_(C) in the internal region to the average crystal grain size D_(F) in the surface region satisfies 1.5<D_(C)/D_(F).
 3. The magnesium alloy material according to claim 1, wherein, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies H_(C)/H_(F)<0.85.
 4. The magnesium alloy material according to claim 1, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 5. A method for producing a magnesium alloy material by rolling a raw material composed of a magnesium alloy, the method comprising: a preparation step of preparing a sheet-shaped raw material obtained by subjecting a molten magnesium alloy to continuous casting by a twin-roll casting process; and a rolling step of rolling the raw material with multiple passes to produce a sheet-shaped magnesium alloy material having a thickness of 1.5 mm or more, wherein a reduction ratio of each of the passes is 25% or less.
 6. The magnesium alloy material according to claim 2, wherein, when a Vickers hardness (Hv) in the surface region is defined as H_(F) and a Vickers hardness (Hv) in the internal region is defined as H_(C), a ratio H_(C)/H_(F) of the Vickers hardness H_(C) in the internal region to the Vickers hardness H_(F) in the surface region satisfies H_(C)/H_(F)<0.85.
 7. The magnesium alloy material according to claim 2, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 8. The magnesium alloy material according to claim 3, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 9. The magnesium alloy material according to claim 4, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less.
 10. The magnesium alloy material according to claim 6, wherein the magnesium alloy contains Al as an additive element in a content of 5.0 mass % or more and 12 mass % or less. 